Method for determining the extent of recovery of materials injected into oil wells during oil and gas exploration and production

ABSTRACT

Disclosed is a method of determining the extent of recovery of materials injected into a oil well comprising the steps of: a) preparing a material to be injected into an oil well; b) admixing therewith a chemical tracer compound at a predetermined concentration; c) injecting the admixture into an oil well; d) recovering from the oil well a production fluid; e) analyzing the production fluid for the concentration of the chemical tracer present in the production fluid; and f) calculating the amount of admixture recovered from the oil well using the concentration of the chemical tracer present in the production fluid as a basis for the calculation. Fluorinated benzoic acids are disclosed as a preferred tracer.

RELATED APPLICATIONS

This application claim priority from U.S. Provisional Patent ApplicationSer. No. 60/293,071 filed on May 23, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for determining the extent ofrecovery of materials injected into an oil well during oil and gasexploration and production. The present invention particularly relatesto a method for determining the extent of recovery of materials injectedinto an oil well during oil and gas exploration and production usingchemical tracers.

2. Background of the Art

The present Invention relates generally to hydrocarbon (oil and gas)production from wells drilled in the earth, hereinafter referred to as“oil wells.” Drilling a hole into the earth to reach oil and gas bearingformations is expensive which limits the number of wells that can beeconomically drilled. It follows then that it is desirable to maximizeboth and the overall recovery of hydrocarbon held in the formation andthe rate of flow from the subsurface formation to the surface, where itcan be recovered.

One way in which to maximize production is the process known asfracturing. Hydraulic fracturing involves literally breaking orfracturing a portion of the hydrocarbon bearing formation surrounding anoil well by injecting a specialized fluid into the wellbore directed atthe face of the geologic formation at pressures sufficient to initiateand/or extend a fracture in the formation. Ideally, what this processcreates is not a single fracture, but a fracture zone, i.e., a zonehaving multiple fractures, or cracks in the formation, through whichhydrocarbon can more readily flow to the wellbore.

Creating a fracture in a hydrocarbon-bearing formation requires severalmaterials. Often these materials, if not removed from the oil well, caninterfere with oil and gas production. Even the drilling mud used tolubricate a drill bit during the drilling of an oil well can interferewith oil and gas production. Taking too long to remove such materialscan increase the cost to the operator of the well by delaying productionand causing excess removal expenses. Not being thorough in removing suchmaterials can increase the cost to the operator of the well throughlower production rates and possible lost production.

Measures taken to remove unwanted or unneeded materials are usuallyinexact. Sometimes additional fluids are used to flush out unwantedmaterials in the well bore. In other situations, reservoir fluids flowcan make estimating return flow very difficult, particularly if thereservoir fluids are incompatible with the injected materials. It wouldbe desirable in the art of oil and gas production to be able todetermine how much of a given material is left in an oil well after adrilling, fracturing or any other operation requiring the injection ofmaterials into an oil well. It would be particularly desirable if such adetermination could be made using an inexpensive and environmentallybenign method.

SUMMARY OF THE INVENTION

In one aspect, the present invention is a method for determining theextent of recovery of materials injected into a oil well comprising thesteps of: a) preparing a material to be injected into an oil well; b)admixing therewith a chemical tracer compound at a predeterminedconcentration; c) injecting the admixture into an oil well; d)recovering from the oil well a production fluid; e) analyzing theproduction fluid for the concentration of the chemical tracer present inthe production fluid; and f) calculating the amount of admixturerecovered from the oil well using the concentration of the chemicaltracer present in the production fluid as a basis for the calculation.

DESCRIPTION OF PREFERRED EMBODIMENTS

As already defined, the term “oil well” means hydrocarbon (oil and gas)production wells drilled in the earth. The method of the presentinvention can also be used with other types of wells that are drilled inthe earth and can require stimulation by hydraulic fracturing, such as awell used for water flooding in secondary recovery operations in oil andgas production. For the purposes of the present invention, the term “oilwell” means hydrocarbon production wells, but also any other type ofwell that can require stimulation by hydraulic fracturing.

In one embodiment, the present invention is a method for determining theamount of fracture materials recovered after the stimulation of an oilwell by means of hydraulic fracturing. Creating a fracture in ahydrocarbon-bearing formation requires several materials. Most oftenthese include a carrier fluid, a viscosifier, a proppant, and a breaker.Other components that are sometimes added include materials to controlleak-off, or migration of the fluid into the fracture face, gelstabilizers, surfactants, clay control agents and crosslinkers.

The purpose of the first fracturing component is to first create/extenda fracture in an oil and gas producing formation and then, once it isopened enough, to deliver proppant. The carrier fluid together withproppant material is injected into the fractured formation. The carrierfluid is simply the means by which the proppant and breaker are carriedinto the formation.

Numerous substances can act as a suitable carrier fluid, though they aregenerally aqueous-based solutions that have been either gelled or foamedor both. Thus, the carrier fluid is often prepared by blending apolymeric gelling agent with an aqueous solution although sometimes thecarrier fluid is oil-based or a multi-phase fluid. Often, the polymericgelling agent is a solvatable polysaccharide, e.g., galactomannan gums,glycomannan gums, and cellulose derivatives. The purpose of thesolvatable or hydratable polysaccharides is to thicken the aqueoussolution so proppant can be suspended in the solution for delivery intothe fracture.

The polysaccharides function as viscosifiers, increasing the viscosityof the aqueous solution by 10 to 100 times, or even more. During hightemperature applications, a cross-linking agent is further added whichfurther increases the viscosity of the solution. The borate ion has beenused extensively as a crosslinking agent for hydrated guar gums andother galactomannans to form aqueous gels, e.g., U.S. Pat. No.3,059,909. Other demonstrably suitable cross-linking agents include:titanium as disclosed in U.S. Pat. No. 3,888,312, chromium, iron,aluminum, and zirconium as disclosed in U.S. Pat. No. 3,301,723. Morerecently, viscoelastic surfactants have been developed which obviatesthe need for thickening agents, and hence cross-linking agents.

Most relevant to the present invention is the final step of thefracturing process. The process of removing the fluid from the fractureonce the proppant has been delivered is referred to as “fractureclean-up.” For this, the final component of the fracture fluid becomesrelevant: the breaker. The purpose of the breaker is to lower theviscosity of the fluid so that it is more easily removed from thefracture.

In another aspect, the present invention is a method for determining theamount of drilling fluid recovered after the completion of an oil well.A drilling fluid is a fluid specially designed to be circulated througha wellbore as the wellbore is being drilled to facilitate the drillingoperation. The circulation path of the drilling fluid typically extendsfrom the wellhead down through the drill pipe string to the drillingface and back up through the annular space between the drill pipe stringand wellbore face to the wellhead. The drilling fluid performs a numberof functions as it circulates through the wellbore including cooling andlubricating the drill bit, removing drill cuttings from the wellbore,aiding in support of the drill pipe and drill bit, and providing ahydrostatic head to maintain the integrity of the wellbore walls andprevent well blowouts.

There are a number of different types of conventional drilling fluidsincluding compositions termed “drilling muds.” Drilling muds comprisehigh-density dispersions of fine solids in an aqueous liquid or ahydrocarbon liquid. An exemplary drilling mud is a dispersion of clayand/or gypsum in water. The solid component of such a dispersion istermed a “weighting agent” and is designed to enhance the functionalperformance of the drilling fluid.

In the practice of the present invention, the extent of recovery ofmaterials injected into a oil well during fracturing, drilling and thelike is determined by preparing the fracture materials or drillingfluids to be injected into an oil well and admixing therewith a chemicaltracer compound at a predetermined concentration. The tracers usefulwith the present invention include any known to those ordinary skill inthe art of using chemical tracers in oil and gas operations to beuseful, but preferably are those which can be detected at concentrationslow enough to make their use economically practical in such operationsand low enough to interfere with the carrier fluid or other materialspresent in the oil well. Preferably the chemical tracers useful with thepresent invention include but are not limited to: fluorinated benzoicacids including 2-fluorobenzoic acid; 3-fluorobenzoic acid;4-fluorobenzoic acid; 3,5-difluorobenzoic acid; 3,4-difluorobenzoicacid; 2,6-difluorobenzoic acid; 2,5-difluorobenzoic acid;2,3-difluorobenzoic acid; 2,4-difluorobenzoic acid; pentafluorobenzoicacid; 2,3,4,5-tetrafluorobenzoic acid; 4-(trifluoro-methyl)benzoic acid;2-(trifluoromethyl)benzoic acid; 3-(trifluoro-methyl)benzoic acid;3,4,5-trifluorobenzoic acid; 2,4,5-trifluorobenzoic acid;2,3,4-trifluorobenzoic acid; 2,3,5-trifluorobenzoic acid;2,3,6-trifluorobenzoic acid; 2,4,6-trifluorobenzoic acid; and the like,perfluoromethylcyclopentane (PMCP), perfluoromethylcyclohexane (PMCH),perfluorodimethylcyclobutane (PDMCB), m-perfluorodimethylcyclohexane(m-PDMCH), o-perfluorodimethylcyclohexane (o-PDMCH),p-Perfluorodimethylcyclohexane (p-PDMCH), perfluorotrimethylcyclohexane(PTMCH), perfluoroethylcyclohexane (PECH), perfluoroisopropylcyclohexane(IPPCH), and the like.

Any chemical compound can be used as tracer with the present inventionif it is not present at a measurable level in the reservoir fluids beingproduced from the well being tested, it can be measured at levelssufficiently low to allow its use to be economical, and the tracer doesnot interfere or interact undesirably with other materials present inthe oil well at the levels used. Preferably, the tracers are detectableat a range of from about 1 parts per trillion to about 10,000 parts permillion in the fluid being analyzed. Preferably the tracers aredetectable at a range of from 5 parts per trillion to about 1,000 partsper million. More preferably the tracers are detectable at a range offrom 100 parts per trillion to about 100 parts per million. Atconcentrations greater than about 1000 parts per million, the use ofsome tracers can become prohibitively expensive or cause unacceptableinteractions with other materials present in an oil well.

The tracers of the present invention are desirably compatible with thefluids wherein they are used. Preferably, the tracer selected is chosento be more compatible with the injected materials than with thereservoir fluids which may recovered concurrently with the injectedmaterials. The fluorinated benzoic acids are particularly preferred astracers for the present invention because they are compatible in bothaqueous fluids as a salt and in organic based fluids as an acid.

In an alternative embodiment of the present invention, more than onetracer can be used to measure multiple operations in the same well. Forexample, oil wells often have more than one producing strata or zone. Inthe practice of the present invention, a fracture job could be done onone strata using a first tracer and a fracture job could be done onanother strata using a second tracer. In recent years, horizontaldrilling has allowed for the drilling of multiple bores terminating in acommon bore which connects to the surface. In multilateral wells such asthese, several different tracers could be used to keep track ofconcurrent recovery of materials from the several legs (lateral bores)of such wells.

In a similar but different embodiment, the method of the presentinvention is used in a process to fracture stimulate multiple intervalsin single or multiple formations, within the same wellbore. This isperformed by: (i) perforating a first interval; (ii) stimulating thatfirst interval; (iii) isolating the first interval, (iv) perforating asecond interval; (v) stimulating the second interval; (iii) isolatingthe second interval; and continuing this pattern. There may be as manyas 12 or 13 such stimulations done on a single wellbore in a shortperiod of time, sometimes only weeks or even days. The operator of thewell will then retrieves the isolation mechanism, typically a bridgeplug, between each interval and begins to clean up all of the stimulatedintervals, often at one time. The method of the present invention isvery useful in such an operation because a different tracer can be usedin each interval and thus can be individually detected during theflowback. The method of the present invention thereby provides anopportunity for a well operator to determine which to what extent eachof the intervals is contributing to the flowback.

In the practice of the present invention, a tracer is admixed with amaterial that is to be injected into an oil well. The tracer can bepremixed with the injection material or it can be admixed as it isinjected. Preferably the tracer is admixed with the injection materialthrough a static mixer as the admixture is pumped into the oil well. Anymethod known to those of ordinary skill in the art of admixing andinjecting materials into oil wells can be used with the method of thepresent invention.

In one preferred embodiment, where a stream of fluids used for ahydraulic fracture job is being pumped into an oil well, a ten percentsolution of a fluorinated benzoic acid salt tracer is pumped into thestream of fluids being used for a hydraulic fracture job, just upstreamof a static mixer, using a peristaltic pump to meter the tracer into thestream of fluids. In another preferred embodiment, the pump used to feedthe tracer solution into the fracture fluids is a triplex or acentrifugal pump. In either embodiment, the metering pump is adjustedsuch that the tracer is injected into the fracture fluids at a rate thatresults in a predetermined tracer concentration appropriate for theconditions in the oil well. The same process can also be used forinjecting tracer into a stream of drilling fluids.

In the practice of the present invention, the chemical tracer compoundis admixed with a material to be injected into an oil well at apredetermined concentration. The concentration of the tracer is aboveits detection limits and preferably at a concentration of ten times itsdetection limits. In the practice of the present invention, preferablythe concentrations of the tracer and the total amount of admixtureinjected is determined and known.

After the fluid injected into an oil well during the practice of thepresent invention has performed its purpose, it is preferably recovered.Most often, the injected materials are recovered along with reservoirfluids as a production fluid. In the practice of hydraulic fracturing ofwells, this phase of the process is the fracture clean up. Inconventional practices, this process can take an extended amount of timewhere up to 72 hours would not be unusual.

In the practice of the present invention, the recovered materials aretested for tracer concentration and the amount of material recovereddetermined. At this point, the well operator can make an informeddecision regarding whether to continue clean up or begin production.

The extent of recovery of materials injected including a tracer of thepresent invention is preferably determined by using a mass balanceapproach. Therein, the total amount of tracer admixed with the injectedmaterial is a known. A homogenous sample of production fluid is testedfor tracer concentration and the amount of tracer recovered is therebydetermined. The amount of injected admixture recovered is thendetermined using the formula:

AMT _(r)=((T _(r) /T _(i))×AMT _(i))

wherein AMT_(r) is the amount of injected admixture recovered, T_(i) isthe amount of tracer injected; T_(r) is the amount of tracer recovered;and AMT_(i) is the amount of materials injected. T_(r) is determined bymultiplying the concentrations of the tracer in the production fluid bythe total quantity of production fluid recovered.

Where a mass balance approach is not possible or desirable, a relativerate of recovery can also be determined by measuring the concentrationof tracer in the production fluids recovered from an oil well as afunction of time. In a process such as this, samples of production fluidbeing recovered from the well are taken, analyzed for tracerconcentration that is then plotted against time and/or flow rates. Thiscan also be a desirable way for an operator to decide when to beginproduction from an oil well.

The tracers used with the method of the present invention can beanalyzed by any method known to those of ordinary skill in the art ofdoing such analyses to be useful. For example, in one method ofanalyzing for a fluorinated benzoic acid tracer of the presentinvention, an emulsion of hydrocarbons, water and naturally occurringinorganic materials is first acidified with dilute hydrochloric acid andthen extracted using a nonpolar solvent. The organic phase is thenadmixed with a 1 normal sodium hydroxide solution and then extractedwith water. The water is then reacidified and extracted with methylenechloride. The recovered methylene chloride is then analyzed for thetracer, optionally after being reduced in volume by evaporation.

In addition to methylene chloride, other solvents can be used. Forexample, cyclohexane, normal hexane, pentane, can be used. While notpreferred, organic solvents such as benzene and toluene can also be usedas long as care is used to make sure that the solvent does not have asignificant background level of the tracer being used.

In the case of the fluorinated benzoic acid tracers, very low levels oftracer can be determined by taking advantage of the carboxylate group tofirst separate the tracer from non-acidic organics as a salt and then,in a second step, concentrate the tracer into an organic solvent byreturning it to its acid form and then extracting it from an aqueousphase.

There are many instrumental methods of analyzing for the tracercompounds useful with the method of the present invention, including butnot limited to, gas chromatography (GC) using flame ionizationdetectors, electron capture detectors, and the like; liquidchromatography (LC); infrared spectroscopy; combination instrumentationsuch as Fourier transform infrared spectroscopy, GC-mass spectroscopy,LC-mass spectroscopy, and the like.

When especially demanding analytical conditions arise, other means ofdoing the analyses can also be used, including using biologically activetracers for immunoassay, preparing functional derivatives of the tracersincluding, for example, esterifacation with more easily analyzedalcohols, and the like.

To achieve low levels of detection, it is necessary that standardlaboratory practices be maintained. Fluids produced from oil wells cancontain hazardous or toxic materials and steps should be taken to ensurethe safety of lab personnel including, but not limited to, avoiding firehazards, scrubbing or removing H₂S and other harmful gasses, andlimiting skin contact with possible carcinogens. Quality assuranceshould be done as with any analytical procedure including using internalstandards, external standards, and the like to ensure the accuracy ofanalyses. Recovery efficiencies can vary from oil well to oil well. Itis important not to overlook simple steps such as accurately measuringsample volumes and filtering irrelevant solids from samples prior toanalysis. Any analytical method that can detect the chemical tracersuseful with the method of the present invention at useful levels can beused with the present invention.

In another embodiment of the present invention, the tracer is in theform of a coating on a solid substrate. In this application, the traceris released gradually into production fluid over time. When co-injectedwith solids such as proppant or pack sand, this use of the tracers ofthe present invention would allow for an estimation of the amount ofco-injected solids in place in the well. If too little tracer weredetected after completion of the injection, or if the tracer leveldecreased too quickly after completion, an oil well operator would knowthat the injected solids were either not properly placed in the well orare being washed out or otherwise being removed from the oil well.

EXAMPLE

The following examples are provided to illustrate the present invention.The examples are not intended to limit the scope of the presentinvention and they should not be so interpreted. Amounts are in weightparts or weight percentages unless otherwise indicated.

A field application of the method of the present invention is performedin an oil and gas well penetrating the Codell formation in Weld County,Colo. A first material (referred to in the art of hydraulic fracturingas a “stage” or, in this case, “the first stage”) is prepared forfracture injection into the well including 0.15 gallons per thousandgallons (gpt) buffer and 1 gpt of GBW23L* which is a high temperatureoxidizing gel breaker, 40 pounds per thousand pounds (ppt) gellingagent, and a first fluorinated benzoic acid tracer; in water. A secondstage is prepared which includes 1-to-2 lbs/gal proppant; 0.15 gptbuffer; 1 gpt of GBW23L; 1 gpt BC31* gel breaker activator which is alow temperature oxidizing breaker activator; 40 ppt gelling agent; asecond fluorinated benzoic acid tracer; and 2.5 (ppt) gel stabilizer, inwater. A third stage is prepared which includes 3 lbs/gal proppant; 40ppt gelling agent; 0.20 gpt buffer; 1 gpt GBW23L; 1 gpt BC31; 1 pptUltra Perm* breaker which is a low temp oxidizing breaker; 1 ppt gelstabilizer; a third fluorinated benzoic acid tracer; and 1.5 ppt gelcrosslinking agent, in water. A fourth stage is prepared which includes4 lbs/gal proppant; 40 ppt gelling agent; 0.20 gpt buffer; 3 ppt GBW5breaker which is a low temp oxidizing breaker; a fourth fluorinatedbenzoic acid tracer, and 1 ppt Ultra Perm.

*GBW-23L, BC31, GBW5 and Ultra Perm are trade designations of BJServices.

Each stage is injected, in turn, under fracture injection conditions.The flow back is tested for the presence and relative concentration ofeach tracer using a GC-mass spectrometer. The comparative amounts oftracer returned are: (A) Fourth fluorinated benzoic acid tracer highestconcentration; (B) Second fluorinated benzoic acid tracer next highestconcentration; (C) First fluorinated benzoic acid tracer next highestconcentration; and (D) Third fluorinated benzoic acid tracer lowestconcentration.

While not wishing to be bound by any theory, it can be concluded thatthe third material injected had the most stable gel structure,effectively locking it into the formation and thus had the lowest flowback and resulting in the lowest recovery of tracer. It can also beconcluded that the fourth material, being last injected and replete withgel breaking materials would have the greatest flowback and thus thehighest recovery of tracers.

What is claimed is:
 1. A method for determining the extent of recoveryof materials injected into a oil well comprising the steps of: a)preparing a material to be injected into an oil well; b) admixingtherewith a chemical tracer compound at a predetermined concentration;c) injecting the admixture into an oil well; d) recovering from the oilwell a production fluid; e) analyzing the production fluid for theconcentration of the chemical tracer present in the production fluid;and f) calculating the amount of admixture recovered from the oil wellusing the concentration of the chemical tracer present in the productionfluid as a basis for the calculation.
 2. The method of claim 1 whereinthe tracer is selected from the group consisting of fluorinated benzoicacids, perfluoromethylcyclopentane (PMCP), perfluoromethylcyclohexane(PMCH), perfluorodimethylcyclobutane (PDMCB),m-perfluorodimethylcyclohexane (m-PDMCH), o-perfluorodimethylcyclohexane(o-PDMCH), p-Perfluorodimethylcyclohexane (p-PDMCH),perfluorotrimethylcyclohexane (PTMCH), perfluoroethylcyclohexane (PECH),and perfluoroisopropylcyclohexane (IPPCH).
 3. The method of claim 2wherein the tracer is a fluorinated benzoic acid.
 4. The method of claim3 wherein the fluorinated benzoic acid is selected from the groupconsisting of including 2-fluorobenzoic acid; 3-fluorobenzoic acid;4-fluorobenzoic acid; 3,5-difluorobenzoic acid; 3,4-difluorobenzoicacid; 2,6-difluorobenzoic acid; 2,5-difluorobenzoic acid;2,3-difluorobenzoic acid; 2,4-difluorobenzoic acid; pentafluorobenzoicacid; 2,3,4,5-tetrafluorobenzoic acid; 4-(trifluoro-methyl)benzoic acid;2-(trifluoromethyl)benzoic acid; 3-(trifluoro-methyl)benzoic acid;3,4,5-trifluorobenzoic acid; 2,4,5-trifluorobenzoic acid;2,3,4-trifluorobenzoic acid; 2,3,5-trifluorobenzoic acid;2,3,6-trifluorobenzoic acid; and 2,4,6-trifluorobenzoic acid.
 5. Themethod of claim 1 wherein the tracer is present in the mixture injectedinto an oil well at a concentration of at least about 1 part pertrillion.
 6. The method of claim 5 wherein the tracer is present in themixture injected into an oil well at a concentration of less than orequal to 10,000 parts per million.
 7. The method of claim 6 wherein thetracer is present in the mixture injected into an oil well at aconcentration of from about 100 parts per trillion to about 100 partsper million.
 8. The method of claim 1 wherein the material injected intothe oil well is a material useful for hydraulically fracturing the oilwell.
 9. The method of claim 1 wherein the amount of injected admixturerecovered is determined using the formula: AMT _(r)=((T _(r) /T_(i))×AMT _(i)) Wherein: (i) AMT_(r) is the amount of injected admixturerecovered, (ii) T_(i) is the amount of tracer injected; (iii) T_(r) isthe amount of tracer recovered; (iv) AMT_(i) is the amount of admixtureinjected; and (v) T_(r) is determined by multiplying the concentrationsof the tracer in the production fluid by the total quantity ofproduction fluid recovered.
 10. The method of claim 1 wherein the traceris in the form of a coating on a solid support.
 11. A method fordetermining the extent of recovery of materials injected into a oil wellcomprising the steps of: a) preparing a material to be injected into anoil well; b) admixing therewith a chemical tracer compound at apredetermined concentration; c) injecting the admixture into an oilwell; d) recovering from the oil well a production fluid; e) analyzingthe production fluid for the concentration of the chemical tracerpresent in the production fluid; and f) calculating the amount ofadmixture recovered from the oil well using the concentration of thechemical tracer present in the production fluid as a basis for thecalculation, wherein the tracer is a fluorinated benzoic acid.